Since the 1990's, the problems of energy crisis and environmental pollution have become increasingly serious worldwide. Petroleum accounts for over 40 percent of worldwide total energy consumption. According to the current proved reserves and consumption speed, it is estimated that the global petroleum resources might run out in the future dozens of years.
Motor vehicles are major consumers of petroleum and a major source of air pollution. Carbon dioxide in the exhaust gas of motor vehicles is one of the main factors for the global greenhouse effect, and other components of the exhaust gas are also chief urban air pollutants.
Practices during the past many years have proven that the use of novel fuel sources and novel power systems are important channels for solving vehicle energy bottleneck and exhaust gas pollution. With respect to novel fuel sources, it has been proven in practice that the use of alternative fuel is cleaner than gasoline and diesel and is a feasible solution for solving energy crisis and pollution caused by emission. With respect to novel power systems, all-electric vehicles employing a power storage battery such as a plumbic-acid storage battery, a Ni-MH storage battery or a lithium storage battery are advantageous in zero emission, low noise and high efficiency. However, due to the restriction of electrical energy storage technologies, all-electric vehicles also have drawbacks in high manufacture costs, short continuous travel mileage and long battery-charging time. Thus, all-electric vehicles cannot really meet people's needs in use and do not have enough market competitiveness at present and for quite some time to come. Hybrid vehicles are a transitional vehicle type between the conventional vehicles and the all-electric vehicles and have advantages of the two types above such as super-low emission, high efficiency and longer continuous travel mileage, and are only slightly more costly than vehicles utilizing a conventional power system.
The power system of a hybrid vehicle includes many types such as a series power system, a parallel power system and a serial-parallel power system, wherein a series hybrid power system exhibits a relatively simple designed structure and good stability and can effectively achieve allocation and synthesis of various energies and is one currently popular technology for researching and developing the power system of hybrid vehicles.
A series hybrid electric vehicle (SHEV) generally uses a power storage battery and a fuel generator set as two kinds of basic energy storage and conventional devices, wherein the fuel generator set (engine+generator) is also called an auxiliary power unit (APU). Generally only one auxiliary power unit is provided in the prior series hybrid vehicle, wherein the auxiliary power unit mainly comprises only one fuel engine (gasoline engine or diesel engine) as an engine and only one generator for converting kinetic energy outputted by the engine into electrical energy. Also, the fuel engine is not mechanically connected to a traction motor driving the vehicle to run. However, while prior series hybrid vehicles can reduce exhaust gas emission of the vehicle and achieve better fuel economy, these vehicles still use gasoline or diesel as fuel and do not address the problem of petroleum resource exhaustion. Current alternative fuels such as natural gas or methane generally have a lower energy density. However, the power system of the series hybrid vehicle generally includes three energy conversion procedures (chemical energy→mechanical energy→electrical energy→mechanical energy), and has a lower energy conversion efficiency than a parallel hybrid power system. Therefore, it is generally believed that alternative fuels with lower energy density are not adapted for use in the series hybrid vehicles.
Functions of the auxiliary power unit of the series hybrid vehicle comprise directly outputting electrical energy and driving the vehicle to run through the traction motor, outputting electrical energy to charge a high-voltage power battery, and charging a vehicle low-voltage storage battery through DC/DC. A basic working mechanism of the auxiliary power unit is such that under the action of a control system, the engine causes a generator rotor to rotate through a mechanical connection, thereby obtaining an appropriate magnetic field intensity in the generator by controlling current intensity in the coil on the generator rotor. In addition, the coil on a generator stator generates electrical energy under action of a generated alternating magnetic field. Before application to vehicle driving, such small and medium-sized fuel generator sets are used as a movable temporary power supply system which is characterized by a relatively stable working state, a smaller rotation speed fluctuation and infrequent start and stop. However, the auxiliary power unit used in the series hybrid vehicle is connected in parallel with the power battery in circuitry and drives the vehicle to run through the motor, which requires the fuel generator set to have characteristics such as large scope of coverage of operating conditions, quick response and smooth transition. The control level of the fuel generator set directly affects power performance, safety and service life of the whole power system. That is to say, the fuel generator set conventionally used as the movable temporary power supply system and the fuel generator set in the series hybrid vehicle have completely different performance requirements, and therefore are different in terms of structural design, system connection and control policy, so that technically they cannot be obviously substituted.
Taking into account both emission and fuel consumption, the auxiliary power unit of the series hybrid vehicle currently has the following two basic working modes: the first is a single-point constant-power working mode (also called a switching mode), and the second is a working mode along a minimum fuel consumption curve.
The auxiliary power unit employing the single-point constant-power working mode only has two states: stably working at a certain working condition point with very low fuel consumption and emission, and a dynamic load of the vehicle is completely balanced by the power battery. In this way, although the emission and fuel consumption of the auxiliary power unit are very low, the power battery may be seriously harmed.
The auxiliary power unit employing the working mode along a minimum fuel consumption curve can track the changes of an actual vehicle load, and effectively reduce requirements of the vehicle for power battery output capacity and power; the power battery may cyclically work with less power, which facilitates prolonging of the service life of the power battery and selection of a power battery with less power. However, the auxiliary power unit must respond quickly to vehicle power requirements, so it will affect efficiency of the engine and emission properties. Furthermore, it is relatively difficult to accurately predict the power needs in a future travel phase of the vehicle.
Additionally, due to limitations of factors such as the number of engine cylinders, cylinder bore and piston stroke, a single engine is limited with respect to power and torque and impossible to expand infinitely, and sometimes cannot meet higher requirements of some vehicles for power and torque. How to meet higher requirements of some vehicles for power and torque is one of the top issues in the vehicle industry.